This invention is primarily concerned with the catalytic cracking of gas oils to produce hydrocarbons of lower molecular weight, such as gasolines, jet fuels, diesel oils, etc. Catalytic cracking of gas oils inherently results in the deposition of significant amounts of a carbonaceous material generally referred to as coke on the catalyst, thereby resulting in a decline of activity of said catalyst which must be compensated for by frequent regeneration of the same by burning off said coke at elevated temperatures in a regenerator. The art is well aware that among the products resulting from the combustion of coke are carbon monoxide and carbon dioxide. The art is also aware that combustion of the carbon monoxide to carbon dioxide generates more heat which, in turn, may be able to be absorbed by the catalyst being regenerated and introduced back into the cracking reaction. This is particularly important since a cracking reaction is endothermic, whereas a regeneration reaction is exothermic. The art is also aware that enhancing the CO oxidation in the regenerator can also have benefits with regard to the fact that the catalyst being regenerated can have its residual coke reduced to such a low level that its activity and selectivity for the catalytic cracking of gas oil becomes considerably enhanced. The art is also well aware of the so-called afterburning phenomenon wherein too high a concentration of carbon monoxide in the regenerator can cause excessive temperatures which can lead to potential damage of both catalyst and equipment. Prior art workers approached the problem of controlling afterburning in a number of ways which can be broadly classified as catalytic techniques and non-catalytic techniques. As examples of non-catalytic techniques of controlling afterburning, one can include such common prior art procedures as utilizing steam injection in order to limit temperature or limiting the amount of carbon monoxide in contact with oxygen such that oxidation of carbon monoxide to carbon dioxide could not take place. Catalytic ways of controlling afterburning include the addition of materials known to have catalytic activity for the oxidation of carbon monoxide to carbon dioxide. One of the earliest materials utilized was chromium oxide and, in fact, a commercial catalyst utilizing chromium oxide to control afterburning was developed and is disclosed in U.S. Pat. No. 2,647,860. However, catalysts useful for the oxidation of carbon monoxide to carbon dioxide are also hydrogenation/dehydrogenation catalysts and, as is well known in the art, it is not desirable to have a hydrogenation/dehydrogenation catalyst in a non-hydrogenative cracking process since these materials usually promote the dehydrogenation of gas oil which leads to excessive coke and hydrogen formation. Thus, until quite recently, the prior art catalysts represented a compromise between the oxidation function which was desirable in order to enhance the conversion of carbon monoxide to carbon dioxide and the hydrogenation/dehydrogenation function which was undesirable.
A dramatic breakthrough in this general area has recently been made and it involves the use of trace amounts of certain Group VIII metals or rhenium. Quite unexpectedly, it was discovered that these Group VIII metals could be used in such low quantities such that they would have a tremendous activity for the oxidation of carbon monoxide, yet they would have substantially little dehydrogenation activity so as not to seriously affect the cracking reaction.
U.S. Pat. Nos. 4,072,600; 4,088,568; and 4,093,535 are patents which disclose and claim the above concept.